Chemical Reaction Dynamics in the Heterogeneous Medium Investigated by Time-Resolved Spectroscopy

Abstract

Chemical reactions are the collections of atomic rearrangements where the reactants transform in the products. The chemical reaction dynamics in solution phase often strongly depend on how the reaction intermediates or products are stabilized by the solvation compared to those of the reactants. In other words, the activation energies of certain chemical reactions are determined based on the solvation of solvent molecules. Chemical and biochemical reactions in physiological environments often occur in the heterogeneous environments, such as inside the confined space of vesicles, in the protein environments, or in the interfacial regions between the phases. In the heterogeneous environments, the solvation of solvent molecules often become limited, which can slow down a certain chemical reaction or variate the reaction mechanisms along the entirely separate pathways. However, experimental investigation of chemical reaction dynamics in the heterogeneous environments are often considered limited in many aspects. Instead, several simple model systems that allow control over the internal environments are often adopted. One of the most suitable model systems for the confined environments of reaction dynamics investigation would be the reverse micelles (RMs), which forms through the self-assembly of surfactant molecules between two solvent phases of discerning polarities. In this dissertation, the RMs have been employed as the reaction media to investigated the chemical reaction dynamics depending on the confined solvent nanopools inside the RMs. It is known that specific atomic rearrangements, such as the breakage or formation of a certain chemical bond, which constitute the chemical reaction often occur on ultrafast time scales although overall chemical reactions may be completed on much longer time scales. Therefore, the investigation of the ultrafast chemical reaction dynamics often lacks the appropriate experimental methods since numerous analytical methods used for the chemical structure investigation only provide the limited time resolutions. In this dissertation, numerous photochemical reactions or processes are to be investigated where the photochemical or photophysical processes in the excited states are initiated by the absorption of pump light with the ultrafast time scales. The separate probe light with the ultrafast time scales can probe the concentrations of the reactants, intermediates, or products by varying the time delays between the pump and probe lights. Intermolecular acid-base reactions and intramolecular charge transfer reactions in the solution phase and inside the nanopools of the RMs have been investigated where the solvent polarity, surfactant charge, or the micelle size dependence of the reaction dynamics will be discussed. The solvation dynamics of confined solvent nanopools inside the RMs will also be discussed to understand the reaction dynamics in the heterogeneous environments of RMs. Chapter 1 describes the basis of the reaction dynamics in heterogeneous environments and the RMs as the model systems of reaction dynamics investigation. Chapter 2 provides the details of the experimental setups of the time-resolved electronic spectroscopic methods used in this dissertation and data analysis. In Chapter 3, the photophysical properties of 1,2-dihydroxyanthraquinone (alizarin) confined in methanol-in-oil RMs investigated by time-resolved fluorescence spectroscopy are summarized. The formation of methanol-in-oil RMs and the location of alizarin molecules inside the nanopools are confirmed by Raman spectroscopy and fluorescence anisotropy measurements. The fluorescence emission of alizarin at 585 nm was significantly enhanced (2.5-3.0 times greater) within the small RMs, along with a marked increase in its fluorescence lifetime compared to bulk solutions. This is attributed to the strong interactions between alizarin and the sulfonate head groups of the surfactant Aerosol OT (AOT) inside the micelle core. These interactions result in a distinct emission band at 585 nm, suggesting that the alizarin molecules are partially deprotonated and closely interact with the anionic surfactant head groups. In Chapter 4, the formation of methanol-in-oil RMs in certain micelle size range investigated by various spectroscopic methods of absorption, emission, Raman, and fluorescence anisotropy measurements are described. The overall results demonstrate that the formation of methanol-in-oil RMs are highly dependent on the concentration of AOT surfactant, and confirm that the methanol-in-oil RMs are stably formed at the higher concentration of AOT over 0.5 M. Furthermore, intramolecular charge transfer (ICT) dynamics of coumarin 481 and coumarin 153 have been discussed. The ICT dynamics of coumarin 481 with rotatable electron donor group show stronger micelle size dependence in the ICT dynamics than coumarin 153 with a rotation-restricted electron donor. In Chapter 5, the intermolecular acid-base (proton transfer) reaction of photoacid, 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (pyranine; HPTS) in confined methanol-in-oil RMs investigated by transient absorption and time-resolved fluorescence spectroscopy are discussed. Notably, HPTS shows substantial deprotonation in the excited state in small methanol-in-oil RMs, while the excited state proton transfer (ESPT) of HPTS does not occur in bulk methanol solution due to the low proton accepting ability of aliphatic alcohols. From the kinetic analysis based on two-step deprotonation model of proton transfer, the rate constant of proton transfer (kPT) is obtained. The kPT of HPTS in the water-in-oil RMs (2.9×109 s-1) strongly decreased from HPTS in bulk water (1.3×1011 s-1), while slight increase is observed in small methanol-in-oil RMs (9.1×108 s-1). The kPT of HPTS in small RMs become similar in the order of ~109 s-1, regardless of the core polar solvent. The slow solvation dynamics inside the nanopools of RMs might be considered as main parameter of the ESPT dynamics of a weak photoacid HPTS, which are similar in both aqueous and nonaqueous RMs, regardless of polar core solvent. The ESPT of photoacids, especially in the nonaqueous RMs, can be crucial in understanding many important chemical reactions involving proton transfer in the confined environments of cells and membranes. In Chapter 6, the ICT dynamics of aminoanthraquinone derivatives in the confined methanol-in-oil Igepal RMs investigated by transient absorption and florescence upconversion spectroscopy are summarized. Inside the RMs, the ICT dynamics of anthraquinone derivatives becomes much slower compared to the bulk methanol solution due to a strongly increased microviscosity inside the RMs. The micelle size dependence of the ICT dynamics varies among three aminoanthraquinone derivatives with amino and methylamino groups, suggesting interplay between the molecular structures especially in the internal rotation of electron donor groups and the microenvironment inside the RMs. Time-dependent density functional theory (TDDFT) calculations supports the experimental results to represent that the activation barriers of the internal rotations along the ICT in the S1 excited states strongly depend on the increased microviscosity inside the RMs. In Chapter 7, the ESPT of a weak photoacid, D-luciferin (pKa* = 0) inside the water-in-oil and methanol-in-oil nonionic RMs investigated by time-resolved electronic spectroscopy will be discussed. The previous works on the ESPT of HPTS in water-in-oil and methanol-in-oil RMs suggest that the proton transfer dynamics inside the nanopools of the RMs would strongly depend on the solvation dynamics of solvent nanopools inside the RMs. The preliminary results on the ESPT of D-luciferin in water-in-oil and methanol-in-oil RMs show solvent-insensitive proton transfer dynamics in small RMs of methanol and water cores, where the proton transfer rate constants kPT on the order of ~109 s-1 comparable to those of the solvation dynamics in small RMs are observed in both methanol-in-oil and water-in-oil RMs. This work may provide insights for further investigations such as how the solvation dynamics inside the nanopools of RMs, nuch slower compared to the bulk, affect the proton transfer dynamics of a weak photoacids.